1. Field of the Invention
This invention relates to a resistive ribbon thermal transfer printing apparatus which uses a resistive ribbon comprising a resistive material layer and a thermally molten ink layer and a plurality of selectively energized electrodes for causing a current to pass through the resistive material layer to cause the ink layer to be selectively molten and transferred to a receiving material such as a paper.
2. Description of the Prior Art
As a thermal transfer printing technology, which is known as a low-cost and high-quality printing technology, resistive ribbon thermal transfer printing technology is known as shown in "Resistive ribbon thermal transfer printing: A historical review and introduction to a new printing technology" by K. S. Pennington, IBM J. RES. DEVELOP. VOL. 29 NO. 5 SEPTEMBER 1985.
The basic method for energizing the plurality of electrodes is to apply voltage pulses of a same pulse width to the electrodes for printing dots at the same time as shown in FIG. 17(a). In FIG. 17(a), the printing data "W" denotes "white" where the corresponding electrode is not energized, and the printing data "B" denotes "block" where the corresponding electrode is energized to print a dot. In this method, however, the flow of the current passed through the part of the resistive material layer under an energized electrode between two adjacent energized electrodes is different from the flow of the current passed through the part under an energized electrode adjacent to an unenergized electrode. The currents caused to flow by two adjacent energized electrodes interfere with each other to be reduced by each other. But the current caused to flow by an energized electrode adjacent to an unenergized electrode passes through a larger area than the area through which the current caused to flow by an energized electrode between two adjacent energized electrode. This causes the printed dots to be not-uniform as shown in FIG. 17(b), which shows a printed image by the pulses shown in FIG. 17(a). In FIG. 17(b), the dots formed by the electrode Nos. 2 and 5 are larger than those formed by the electrode Nos. 3 and 4.
To solve the above problem, a time-divisional energizing method was introduced as shown, for example, in Japanese Laid-Open patent application No. 59-167279. In this method, a plurality of electrodes are divided into blocks and the electrodes in each block are energized time-divisionally by time-divisional pulses as shown in FIG. 18. This method can solve the above problem of not uniform printed image, but has some new problems. One problem is that the printing speed becomes low due to the time-divisional driving. Another problem is that the linearity of the printed image becomes worse because the different electrodes are energized at different timings. Still another problem is that the resistive ribbon would be damaged due to a shock of a large pulse current flown through a small area in a short period.